1. Field
The present invention relates to a method of initializing an information recording medium by irradiating light beams, and particularly to a method for a rewritable phase-change optical recording layer that is capable of giving rise to a phase change to achieve repeated read-write operations by laser beam irradiation.
2. Description
Optical recording media have recently come into wide use as a viable information storage and archival means of large capacity. Among these media, demands have increased for a phase-change optical recording medium for use in a CD-R disk, for example, because of its repeatable (or writable) read/write capability.
The phase-change optical recording medium is generally provided on a disk-shaped substrate with several layers successively formed by, for example, the sputtering method, such as a first dielectric layer, a recording layer, a second dielectric layer and a layer either of metal or alloy, in the order recited. In addition, a layer composed of ultraviolet curing resin is formed thereon by spin coating, for example.
Since the thus formed recording layer is in an amorphous state, process steps of crystallization (i.e., “initialization” steps) are to be carried out before recording onto the recording layer by a recording system or disk drive.
During the process steps, the recording layer is once heated to a temperature above its melting temperature. When the recording layer is then cooled relatively slowly, it transforms into a crystalline state. It transforms into an amorphous state when cooled rapidly.
As a suitable material for forming the substrate, polycarbonate resin is frequently used for its excellent properties such as mechanical stability, light weight and low costs, among others.
When the entire recording disk is initialized all at once, the allowable temperature of the polycarbonate substrate may be exceeded by heat from a large power density of light beam irradiation. Therefore, the initialization steps are generally carried out gradually over the disk area, in which disk portions are successively irradiated by a focused intense beam from a laser device along with the rotation of the disk.
To be more specific, the initialization of an optical disk is achieved by providing a laser device with its longer axis of the power distribution perpendicular to disk tracks and by imparting a gradual displacement of the laser device along the radial direction of the disk (i.e., perpendicular to the direction of disk tracks).
When a large fluctuation in the laser power exists along the longer axis of the laser power distribution perpendicular to disk tracks, reflectivity values of the optical disk after the initialization may undesirably fluctuate along the perpendicular direction. This may result in scatter in tracking signals, which is a drawback throughout succeeding reading and signal processing steps of the optical disk.
In addition, initialization steps using the above described background art laser device and optical system take a relatively long time to complete, since the optical disk is irradiated successively, portion-wise along with the rotation of the disk and cooled relatively slowly.